Zoom camera

ABSTRACT

In a zoom camera having a zoom lens including a variator lens, and a driving system for driving the variator lens, the image magnification is computed in accordance with a predetermined program based on the detected object distance, and such a focal length is computed as to obtain a specified value of image magnification based on the detected object distance and the computed image magnification, and the obtained focal length data is fed to the driving system. As a result, auto-zooming with a suitable image magnification in accordance with the object distance can be achieved.

This application is a continuation of application Ser. No. 07/231,438,filed Aug. 12, 1988 now abandoned.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a zoom camera in which the imagemagnification is variable in accordance with the object distance.

Cameras having a zoom lens system capable of photographing with constantimage magnification are conventionally known.

For example, in an apparatus shown in Japanese Laid-Open PatentPublication (Tokkaisho) No. 53-113527, a position of a variator lensgroup with which images of a constant image magnification can beobtained even if the object distance changes is computed and set basedon the object distance and the image magnification.

Further, in an art shown in Japanese laid-Open Patent Publication No.55-64204, the rotation angles and the divisions of scales of a focusring and a zoom ring respectively are so designed that the two rings canintegratedly rotate so as to obtain a constant image magnification. Inthis prior art publication, it is described that if slightly differentimage magnifications, not an absolutely fixed one, are preferred, otherdesigns can be made in order to obtain such image magnifications.However, such designs have not been disclosed concretely there.

Further, in the abovementioned two prior arts, no range of the objectdistance wherein constant image magnification can be obtained has beendisclosed.

Further, in an art shown in Japanese Laid-Open Patent Publication No.54-39144, a compensator lens group can make a predetermined movementdifferent from the movement of a variator lens group so thatphotographing with constant image magnification can be achieved only bythe zooming operation. In this prior art publication, it is describedthat a constant image magnification is obtained in the range of thephotgraphing distance D from 1.5 m to 8.8 m. However, how to treat theimage magnification at the photographing distance D below and above thisrange has not been disclosed.

Further, in an art shown in Japanese Patent Publication (Tokkosho) No.51-27577, a focus ring and a zoom ring are interlocked with each otherby providing a zoom cam between the two rings which meets the relationrepresented by a formula M=f/x (M=image magnification, x=photographingdistance; f=focal length of zoom lens) so that photographing withconstant image magnification can be achieved only by the focusingoperation. In this prior art published, it is described that a constantimage magnification is obtained in the range of the photographingdistance D from 2 m to 10 m. However, how to treat the imagemagnification at the photographing distance D below and above this rangehas not been disclosed.

Further, in an art shown in Japanese Laid-Open Patent Publication No.61-38917, the focal length f of a zoom lens is automatically controlledso as to make the value f/D to be constant. And if the photographingdistance D is above a range in which zooming is executed interlockedlywith auto-focusing, the focal length is controlled to be at the closestzoom position and an indication OUT OF RANGE is displayed.

Further, in an art shown in Japanese Laid-Open Utility Model PublicationNo. 62-69224, when the photographing distance detected based on adistance measuring signal becomes large, zooming up (closeup) isexecuted, and when the photographing distance becomes small, zooming hasno relation with the photographing distance.

Further, in an art shown in Japanese Laid-Open Patent Publication No.60-4911, a focusing mechanism and an image magnification changingmechanism are mechanically interlocked, so that the image magnificationis set to be high on the longest focal length condition and low on theshortest focal length condition. And an embodiment is described in whicha zoom lens with a focal length from 80 mm to 200 mm is used and thefocusing mechanism and the image magnification changing mechanism are sointerlocked that when the focal length is 80 mm, an object at the objectdistance of 20 m is in in-focus and when the focal length is 200 mm, anobject at the object distance of 40 m is in in-focus.

Further, in an art shown in Japanese Laid-Open Patent Publication No.62-118328, pictures having a suitable size of object image are obtainedbased on distance data. And an example is shown in which when an objectto be photographed is at a far distance or at a middle distance, a focallength setting switch is turned to a longer focal length, and when anobject is at a close distance, it is turned to a shorter focal length.

Futhermore, in an art shown in Japanese Patent Publication No. 60-1602,the focal length is controlled in a feed-back manner by treatinglogarithms of the constant image magnification, the photographingdistance and the focal length.

In this connection, zooming has a relation to the framing of images tobe photographed, and it is judged by a print or a negative film whethera zooming is good or bad. However, generally it is very difficult for auser to judge at what focal length position to stop zooming operation.Accordingly, if basic image magnification is proposed to users bysetting focal lengths variable according to predetermined programrelating to a photographing distance, a more desirable picture can betaken by an easy operation in auto-zooming. And as a result, zoomingbecomes truly easy to use.

SUMMARY OF THE INVENTION

The present invention is based on the abovementioned background. Anobject of the present invention is to provide a zoom camera which canmake programmed auto-zooming operation throughout the whole range of thephotographing distance, so that a user can concentrate his attention onshutter release opportunities and pictures having a suitable size ofobject image can be obtained.

According to the present invention, a camera having an auto-zoomingsystem is provided in which an image magnification setting means forsetting the image magnification in accordance with a predeterminedprogram based on the object distance and the focal length is computedbased on a value of the image magnification setting means and thedetected object distance so as to obtain a specified imagemagnification, whereby lens is driven based on the computed focallength. Accordingly, zooming at a suitable image magnification inaccordance with an object distance can be executed, pictures having asuitable size of object image can be easily obtained, which isadvantageous especially for taking snapshots and portraits. And by theautomatization of zooming, a user can concentrate his attention only onseizing shutter release opportunities.

Further, by setting the image magnification in the case of an objectbeing near larger than that in the case of an object being far, thephotographic purpose of a beginning user can be more apparentlyrepresented in a picture, to increase the convenience to use the zoomcamera.

Further, it is judged whether the focal length computed asabovementioned is within the focal length range of the zoom lens, andwhen it is within the range, the zoom lens is driven to have thecomputed focal length, and when it is outside the range, the zoom lensis driven with setting the focal length at one end of the focal lengthrange of the zoom lens. Consequently, auto-zooming at a suitable imagemagnification in accordance with an object distance can be achieved inphotographing within the range of usual object distance.

Further, photographing on the near side and the far side outside thefocal length range of the zoom lens can be executed at the shortestfocal length and the longest focal length of the zoom lens respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the arrangement of a zoom camera of anembodiment of the present invention;

FIG. 2 is a program diagram of the image magnification in relation tothe object distance and the focal length in the embodiment of thepresent invention;

FIG. 3 is a view showing an optical system section for explaining aprinciple of auto-zooming program;

FIG. 4 is a flow chart for executing the auto-zooming program;

FIG. 5 is a program diagram of another embodiment of the presentinvention; and

FIG. 6 is a flow chart for executing the program of FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

According to the present invention, a zoom camera having a zoom lensincluding a variator lens group, and a driving system for driving thevariator lens group, comprises an object distance detecting means fordetecting the distance from the camera to an object to be photographed,an image magnification setting means for setting the image magnificationin accordance with a predetermined program based on the object distance,and a computing means for computing such a focal length as to obtain aspecified value of image magnification based on output signals from theobject distance detecting means and the image magnification settingmeans and feeding the computed focal length data to the driving system.

With this arrangement, the image magnification varies in accordance witha specified program based on the object distance.

According to the present invention, a zoom camera comprises, in additionto the abovementioned arrangement, a judging means for judging whetherthe focus length computed by the computing means is within the focallength range of the zoom lens, and means for setting the focal length atone end of focal length range of the zoom lens when the focal lengthcomputed by the computing means is judged not to be within the focallength range of the zoom lens, and the driving means is driven by outputfrom the computing means or the focal length setting means.

With this arrangement, in photographing within the focal length range ofzoom lens, the image magnification varies in accordance with specifiedprogram based on the object distance, and in photographing outside theabovementioned focal length range, the focal length is set at one end ofthe focal length of the zoom lens.

Now, embodiments of the present invention will be described withreference to the appended drawings.

FIG. 1 shows an arrangement of an embodiment of the present invention.In FIG. 1, a photographing zoom lens is constituted of lens group L_(F),L_(Z1), L_(z2), and L_(M), L_(F) indicates a focusing lens group. Bymoving the focusing lens group on the optical axis, objects in positionsfrom the infinitely far position to the nearest photographing position,can be focused. L_(Z1), L_(Z2) indicate a zooming lens group or avariator lens group. By moving the variator lens group L_(Z1), L_(Z2) onthe optical axis in specified manners respectively, the focal length ofthe zooming lens varies from the shortest one to the longest one. L_(M)indicates a master lens group used for focusing images on a film planeF. A focus detecting section 2 receives a branched bundle of ray passingthrough exit pupil of the photographing zoom lens and converts analoguedata indicative of an amount of received light into digital data to feedthem to a microprocessor 1.

A focus motor driving circuit 3 controllingly drives a motor 4 inaccordance with a rotation amount signal fed from the microprocessor 1to move the focusing lens group L_(F) in the direction of the opticalaxis thereof. At this time, an encoder 5 monitors the rotation of themotor 4, and feeds a pulse each time the motor 4 is rotated by apredetermined rotation to the focus motor driving circuit 3. The focusmotor driving circuit 3 counts the pulses fed from the encoder 5 andstops the drive of the motor 4 when the pulses fed from the encoderreaches the value corresponding to the rotation amount signal fed fromthe microprocessor 1. A counter 6 counts pulses fed from the encoder 5generated after the focusing lens group L_(F) is driven from the lensretraction position (∞) and feeds the count value to the microprocesser1.

A zoom motor driving circuit 7 receives signals from the microprocessor1 to drive and control a motor 8 and controls a zoom operating section9. The zoom operating section 9 moves the variator lens group L_(Z1),L_(Z2) to a specified position by means of moving members such as cams,gears or the like connected to a holding member for holding the variatorlens group Lz₁, Lz₂. The zoom operating section 9 has a brush BR whichcontacts with a code pattern formed on a code plate CD. The brush BR andthe code plate CD constitute an encoder. The encoder issues digitalcoded data representing the position of the zoom operating section 9,that is, the present focal length of the zoom lens and feeds the digitaldata to a memory section 10. The memory section 10 is constituted byROM, which stores focal length data in addresses designated by thedigital data and feeds the focal length data stored in the designatedaddress to the microprocessor 1. Further, the memory section 10 storesother lens data peculiar to the camera lens, for example, rotationamount converting data (Δd/ΔL), the longest focal length data (f_(F)),and shortest focal length data (f_(N)).

A setting section 11 feeds a mode signal of either usual zoomingoperation or programmed auto-zooming operation based on a manualoperation to a mode switch S1, to the microprocesser 1. In usual zoomingoperation mode, a manual signal commanding a manually zooming operationto a longer or shorter focal length direction based on a manualoperation to zooming direction switches (not shown) is fed through themicroprocessor 1 to the zoom motor driving circuit 7 to drive the motor8. On the other hand, in programmed auto-zooming operation mode, thebelowmentioned specified computation is executed in the microprocessor1, and a control signal is fed to the zoom motor driving circuit 7 tocontrollingly drive the motor 8.

Now, the output and input data of the microprocessor 1 will be describedin the following.

Firstly, the operation mode of the setting section 11 is detected at theinput terminal i₁. In other words, in programmed auto-zooming operationmode, the mode switch S1 of the setting section 11 is turned ON and alow level signal is fed to the input terminal i₁. On the other hand, inusual zooming operation mode, a high level signal is fed thereto.Further, lens data peculiar to the zoom lens and focal length datavarying momentarily in accordance with the zooming operation, aresuccessively transferred from the memory section 10 to an input terminali₂ to be successively stored in a memory of the microprocessor 1. Anddata Δd/ΔL, f_(N), f_(F), the present focal length data f are stored inmemories M₂₁, M₂₂, M₂₃ and M₂₄ respectively in this order. Further,digital data from the focus detecting section 2 are fed to an inputterminal i₃. In accordance with the data input to terminal i₃, a defocusamount and the direction thereof are computed in the microprocessor 1.Further, in accordance with the defocus data and the data Δd/ΔL storedin the memory M₂₁, the rotation amount is computed. And the count valuen of the counter 6 is fed to an input terminal i₄.

The microprocessor 1 computes the object distance D from theabovementioned count value n. In other words, the relations between theextension amount x' of the lens, the object distance D and the focallength f are approximately represented by the following formula.

    f.sup.2 /D≈x'

Here, the count value n of the counter 6 which counts pulses of theencoder 5 from the most retracted position (∞) to the present positionof the camera lens and the extension amount x' of the lens are generallyin proportion to each other and the following formula is obtained.

    n=ax' (a=constant)

From theses formulas, the following formula can be obtained.

    1/D=(1/f.sup.2 a)·n

When a coefficient determined by each of the camera lens is representedby k, the data k/f² a is peculiar to the camera lens. Consequently, thisdata is held in the memory section 10 and fed through the input terminali₂ into the microprocessor 1 to be stored in a memory M₂₇. In otherwords, in the microprocessor 1, the object distance D is computed bysubstituting the data k/f² a stored in the memory M₂₇ and the countvalue n of the counter 6 fed to the input terminal i₄ into theabovementioned formula, and the computed object distance D is stored ina memory M₄₀.

Now, relating to the output data, the focal length f_(P) is computedbased on the object distance D stored in the memory M₄₀, and the programdiagram f_(P) =f(D) (described below with reference to FIG. 2), and thecomputed focal length f_(P) is stored in the memory M₁₀. The differencebetween the focal length f_(P) stored in the memory M₁₀ and the presentfocal length f stored in the memory M₂₄ is computed and this differenceis issued as a zoom motor driving rotational amount signal from anoutput terminal P₁. And a focus motor driving rotational amount signalis transferred from an output terminal P₂ to the driving circuit 3.

Now, the auto-zooming program will be described. FIG. 2 is a graphshowing the relation between the object distance D and the focal lengthf using the image magnification β as a parameter. When D and f change ona chain line, the image magnification β=f/D becomes constant.

Here, it will be described with reference to FIG. 3 that the formulaβ=f/D is realized in the optical system shown in FIG. 1.

In FIG. 3, the following formula is obtained.

    D=x+2f.sub.F +HH'+x'+x"

Here, since x=f_(F) /β_(F), x=-f_(F) β_(F), the abovementioned formulais represented by the following formula. ##EQU1## Here, in the case ofβ<1/10, since 2+1/βF-β_(F) ≈1/β_(F), HH'<<D, x"<<D, the followingformula is obtained.

    D=f.sub.F /β.sub.F

Further, since f=f_(F) β_(z), β=β_(F) β_(z), a formula D=f/β isobtained. When the image magnification of a zoom lens is not extremelylarge, that is, in usual photographing such as taking a portrait or asnapshot, the relation β=f/D is realized.

In FIG. 3, the indications are as follows.

L_(F) : focusing lens group

f_(F) : focal length of focusing lens

HH': axial distance between principal points of focusing lens

β_(F) : magnification of focus lens (not shown)

L₂ : variator lens

L_(M) : master lens:

β_(Z) : magnification of variator lens and master lens

F,F': focal position of focusing lens

A: position of object point

B: position of image focused by focus lens

D: distance from film plane to object (object distance)

x": distance from image point of focusing lens to film plane

f: focal length of whole system (not shown)

β: image magnification of whole system (not shown)

Now, returning to FIG. 2, the full line is a program diagram of anembodiment of the present invention. The focal length range isrepresented by a range from one end f_(F) mm on the tele side to theother end f_(N) mm on the wide side. When an object to be photographedis in a position with an object distance smaller than D_(N) m, it is ona line with the fixed focal length f_(N) mm (line A in FIG. 2), and theimage magnification largely varies. When an object is in a position withan object distance between D_(N) and D_(F), the focal length varies witha specified relation to the image magnification based on with the objectdistance. In other words, when the object distance is D_(N) on the nearside, the focal length is set at the wide side end f_(N) to make theimage magnification β_(N) =f_(N) /D_(N). And when the object distance isD_(F) on the far side, the focal length is set at the tele side endf_(F) to make the image magnification β_(F) =f_(F) /D_(F). The relationbetween β.sub. N and β_(F) is β_(N) ≠β_(F) and β_(N) >β_(F). In otherwords, when the object is on the near side, a large-size of object imageis obtained in a picture, and when the object is on the far side, asmall-size of object image is obtained in a picture. However, the imagemagnification does not so much vary as the object distance varies withthe focal length being fixed, but it varies within a small range about afixed value. The change of the image magnification is such that, inphotographing a portrait, the upper half of a human body is photographedin a picture when an object is on the near side and the whole body isphotographed when the object is on the far side. A line connectingpoints D_(N) and D_(F) at which β_(N) >β>β_(F) is line B in FIG. 2. Whenan object is in a position having an object distance larger than D_(F),the image magnification β largerly varies on a line C with the fixedfocal length f_(F) mm.

Now, the computation of a program line view f_(p) =f(D) shown in FIG. 2will be described in the following. It is considered that the line Brepresents a formula f_(P) =mD+n. Here, since

    m=(f.sub.F -f.sub.N)/(D.sub.F -D.sub.N)

    n=(f.sub.N D.sub.F -f.sub.F D.sub.N)/(D.sub.F -D.sub.N).

and f=βD, m and n can be represented as follows by using β_(N), β_(F),D_(N) and D_(F).

    m=(β.sub.F D.sub.F -β.sub.N D.sub.N)/(D .sub.F -D.sub.N)

    n={D.sub.N D.sub.F (β.sub.N -β.sub.F)})}/(D.sub.F -D.sub.N)

Line A represents f_(p) =f_(N) and line C, f_(P) =f_(F). They are alsof_(P) =β_(N), and f_(P) =β_(F) D_(F).

Now, an example of operation of the microprocessor 1 executing theabovementioned auto-zooming program will be described with reference toa flow chart of FIG. 4.

#10; Focus condition detection is executed based on data from the focusdetecting section 2. When the focusing lens group L_(F) comes to an inin-focus position, the microprocessor 1 stops the rotation of the motor4. At this time, a count value n of the counter 6 is fed to the inputterminal i₄.

#15; Mode of zooming operation is judged. In usual zooming operationmode, i₁ =1 and a manual signal representing a direction of rotationdesignated by a manually operating section 11 is fed from P₁ to themotor driving circuit 7 to drive the motor 8 in that direction (#20,#25). When i₁ =0, the program goes in programmed auto-zooming operationmode beginning from step #30.

#30; The object distance D in the in-focus state is computed based onthe count value n of the counter 6 fed to the input terminal i₄ and thevalue 1/f² a of the camera lens fed to the input terminal i₂ and storedin the memory M₂₇. The value D obtained by the computation is stored inthe memory M₄₀ (#35).

#40; The focal length value f_(P) on line B in the program diagram ofFIG. 2 is computed according to the formula f_(P) =mD+n and the objectdistance D stored in the memory M₄₀. The obtained value f_(P) is storedin the memory M₁₀ (#45).

#50; Then, it is judged whether the value f_(P) stored in memory M₁₀ islarger than the shortest focal length f_(N) fed to the input terminal i₂and stored in the memory M₂₂.

#55; When f_(P) <f_(N), f_(P) is set to be f_(N). f_(N) instead of mD+nis stored in the memory M₁₀ (#60).

#65; When f_(p) >f_(N), then f_(P) is similarly compared with thelongest focal length f_(F) at the tele side end stored in the memoryM₂₃. When f_(p) ≦f_(F), f_(P) is set to be mD+n computed at Step #40 andit is stored in the memory M₁₀.

#70; When f_(P) >f_(F), f_(P) is set to be f_(F) and f_(F) instead ofmD+n is stored in the memory M₁₀ (#75).

#80; A signal of the difference between f_(P) stored in the memory M₁₀at steps #40 to #75 and the focal length f at the present time fed tothe input terminal i_(z) and stored in the memory M₂₄ is fed through theoutput terminal P₁ to the motor driving circuit 7. At this time, whenthe signal (M₁₀ -M₂₄) is positive, the motor 8 is driven in thedirection increasing the focal length, and when negative the motor 8 isdriven in the direction decreasing the focal length.

#85; The difference between f_(P) stored in the memory M₁₀ and thepresent focal length f momentarily varying in accordance with therotation of the motor 8 fed through the input terminal i₂ and stored inthe memory M₂₄ is detected all the time. When the difference is not 0,the signal (M₁₀ -M₂₀) is continuously issued from P₁ at step #80. Whenthe difference becomes 0, the present focal length becomes apredetermined value f_(P), and accordingly a signal for stopping therotation of the motor 8 is issued from P₁ to stop the motor 8 (#90).

#95; After series of operation is completed at steps #10 to #90 then, itis judged whether i₁ =0 or 1. When i₁ =1, the program is in usualzooming operation mode, and the program advances to step #20. When i₁=0, the program is in programmed auto-zooming operation mode, and thenadvances to step #10, At step #10, focus condition detecting operationand auto-focusing operation are executed.

Now, another embodiment of the present invention will be described inthe following.

In the abovementioned embodiment, the value f_(P) stored in the memoryM₁₀ is compared with the present focal length f fed to the inputterminal i₂ of the microprocessor 1 based on the digital code issuedfrom the encoder (BR and CD) by the drive of the motor 8. However, thisstep can be executed in the following manner. The output of the encoder(BR and CD) is fed as a monitor signal to the motor driving circuit 7.And when the rotation amount in correspondence with the differencebetween the present focal length f issued from the output terminal P₁and fed to the motor driving circuit 7 and the value f_(P) in theprogram diagram becomes the rotation amount in accordance with theencoder (BR and CD), the motor driving circuit 7 stops the rotation ofthe motor 8. In other words, step #80 in the flow chart of FIG. 4 isexecuted in the motor driving circuit 7.

Further, in the abovementioned embodiment, two points β_(N) and β_(F) online B in the program diagram of FIG. 2 are connected by a straightline. However, they can be connected by a curved line. In the case of acircle, (D-a)² +(f-b)² =r². And further, curved lines such as a parabolaand hyperbola can be used.

Now, another program diagram will be described with reference to FIG. 5.In this embodiment, the image magnification is set to be large with theobject distance being on the near side, and to be small with it being onthe far side, similarly to the case of the program line diagram view ofFIG. 2. However, in FIG. 5, a line in correspondence with line B of FIG.2 is divided into five segments B1 to B5 and then connected stepwisetogether.

Relating to segment B1, the object distance varies from D_(N) to D₁,with the image magnification being a fixed value β₁, the focal lengthvarying from f_(N) to f_(A).

Segment B2; the object distance D₂ -D₃, the image magnification β₂, andthe focal length f_(A) -f_(B), and

Segment B3; the object distance D₄ -D_(F), the image magnification β₃and the focal length f_(B) -f_(C).

The relation of the image magnifications with one another is β₁ >β₂ >β₃.

Segment B4; the object distance D₁ -D₂, the image magnification β₁ -β₂,and the focal length f_(A) (fixed), and

Segment B5; the object distance D₃ -D₄, the image magnification β₂ -β₃and the focal length f_(B) (fixed).

The program diagram of FIG. 5 is set as abovementioned.

The arrangement for executing this program is the same as that ofFIG. 1. However, the data f_(A), f_(B) at the turning points of theprogram diagram are transferred from the memory section 10 to the inputterminal i₂ and stored in the memories M₂₅, M₂₆. An algorithm in themicroprocessor 1 is executed according to a flow chart of FIG. 6. Steps#10 to #35 and #80 to #95 are the same as those of FIG. 4. Step #40 to#72 in FIG. 6 will be described in the following.

#40; In order to judge on which of segments A to C the object distance Dstored in the memory M₄₀ is, the value f_(P) is computed according tothe formula f_(P) =β₂ D, and stored in the memory M₁₀ (#41).

#45; It is judged whether f_(P) stored in the memory M₁₀ is above theuppermost value f_(B) of the focal length of segment B2 or not. Whenf_(P) is not above f_(B) and at the same time is not above the lowermostvalue f_(A) at Step #60, f_(P) according to the formula f_(P) =β₂ D isstored in the memory M₁₀. Then the program advances to step #80.

#46-#50; When f_(P) is above the uppermost value f_(B) at step #45, theprogram advances to #46. f_(P) is computed according to the formulaf_(P) =β₃ D relating to segment B3. Then, it is judged whether f_(P) isabove the longest focal length value f_(F) of an interchangeable zoomlens or not. When f_(P) is above f_(F), f_(P) is on line C, andtherefore f_(P) is set to be f_(F) at steps #51, 52 and this value isstored in the memory M₁₀. Then the program advances to step #80.

#55; When f_(P) is not above f_(F) at step #50, f_(P) is then comparedwith f_(B). When f_(P) is above f_(B), the image magnification incorrespondence with the object distance D is on segment B3.Consequently, f_(P) according to the formula f_(P) =β₃ D is stored inthe memory M₁₀ and the program advances to step #80. When f_(P) is notabove f_(B), the program advances to step #56.

#56, #57; When f_(P) is not above f_(B) at step #55, this is a case inwhich f_(P) in the formula f_(P) =β₂ D at step #45 is above or equal tof_(B). Consequently, f_(P) is considered to be on segment B5 and f_(P)is set to be f_(B). This value is stored in the memory M₁₀ and theprogram advances to step #80.

#61-#65; When f_(P) in the formula f_(P) =β₂ D is below f_(A) at step#60, f_(P) is computed according to the formula f_(P) =β₁ D (#61). Theobtained value f_(P) is stored in the memory M₁₀ (#62). Then, it isjudged whether f_(P) is above the shortest focal length value f_(N) ofthe interchangeable lens or not (#65). When f_(P) is below f_(N), it ison line A. Consequently, f_(P) is set to be f_(N) and this value isstored in the memory M₁₀ at steps #66, #67. The program advances to step#80.

#70; When f_(P) is above or equal to f_(N) at step #65, f_(P) is thencompared with f_(A). When f_(P) is not above f_(A), the imagemagnification in correspondance with D is considered to be on segmentB1, and the program advances to step #80.

#71, #72; When f_(P) is above f_(A) at step #70, this is considered tobe in a flow in which f_(P) in the formula f_(P) =β₂ D is below f_(A) atstep #60, and to be on segment B4. Then, f_(P) is set to be f_(A), andthis value is stored in the memory M₁₀. The program advances to step#80. Steps following to step #80 are the same as those of FIG. 4.

While the embodiments of the present invention, as herein disclosed,constitute a prefferred form, it is to be understood that other formsmight be adopted.

What is claimed is:
 1. A zoom camera comprising, a zoom lens including avariator lens group and having a focal length range, a driving systemfor driving the variator lens group, object distance detecting means fordetecting an object distance from the camera to an object to bephotographed, image magnification setting means for setting the imagemagnification in accordance with a predetermined program based on theobject distance, the image magnification being set in such a manner thatan image magnification with a small object distance is larger than animage magnification with a large object distance, and computing meansfor computing such a focal length so as to obtain a specified value ofimage magnification based on output signals from the object distancedetecting means and the image magnification setting means and feedingthe obtained focal length data to the driving system.
 2. A zoom cameraas claimed in claim 1 further comprising, judging means for judgingwhether the focal length computed by the computing means is within thefocal length range of the zoom lens, and means for setting the focallength at one end of the focal length range when the focal lengthcomputed by the computing means is judged not to be within the focallength range of the zoom lens, whereby the driving system is driven byoutputs of the judging means.
 3. A zoom camera as claimed in claim 2, inwhich the image magnification is set by the image magnification settingmeans using such a function as to make the image magnificationassociated with a small object distance larger than the imagemagnification associated with a large object distance, and the means forsetting the focal length is so constructed as to set the focal length atthe shortest focal length of the zoom lens when the image magnificationis large, and at the longest focal length thereof when the imagemagnification is small.
 4. A zoom camera as claimed in claim 1, whereina rate of change of the image magnification is slower when the focallength data is within said focal length range than when the focal lengthdata is at the outer limits of said range.
 5. A zoom camera as claimedin claim 1, in which the change in focal length is determined inaccordance with a predetermined program relating the image magnificationto the object distance and the focal length, wherein a rate of change ofthe image magnification in the case of the object distance being changedwhen the focal length is changed based on said program is slower than arate of change of the image magnification in the case of the objectdistance is changed when the focal length is fixed.
 6. A zoom cameracomprising:a zoom lens including a variator lens group; object distancedetecting means for detecting an object distance from the camera to anobject to be photographed; means for inputting the detected objectdistance and for outputting a focal length, wherein the relationshipbetween the inputted object distance and the focal length to beoutputted is predetermined in such a manner that an image magnificationwith a small object distance is larger than an image magnification witha large object distance; and means for driving the variator lens groupbased on the outputted focal length.
 7. A zoom camera as claimed inclaim 6, wherein a rate of change of the image magnification within apredetermined object distance range in the case of the focal lengthbeing changed by said inputting and outputting means is slower than arate of change of the image magnification in the case of the focallength being fixed.